This is class of turbulence models for the RANS equations in which an eddy viscosity coefficient is used to relate the mean turbulence field to the mean velocity field, however in a nonlinear relationship

where

is a nonlinear function possibly dependent on the mean strain and vorticity fields or even other turbulence variable

is the coefficient termed turbulence "viscosity" (also called the eddy viscosity)

is the mean strain rate

is the mean vorticity

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